Why You Need Several PHP Versions

On a server with several sites you often need different PHP versions: one WordPress site requires PHP 8.1+, while another — a legacy Magento 1 — only runs on PHP 7.4. The ondrej/php repository lets you keep several versions at the same time.

Adding the ondrej/php Repository (Ubuntu/Debian)

sudo apt install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt update

Installing Several PHP Versions

# PHP 7.4
sudo apt install php7.4-fpm php7.4-cli php7.4-mysql php7.4-curl \
    php7.4-mbstring php7.4-xml php7.4-zip php7.4-gd

# PHP 8.1
sudo apt install php8.1-fpm php8.1-cli php8.1-mysql php8.1-curl \
    php8.1-mbstring php8.1-xml php8.1-zip php8.1-gd

# PHP 8.2
sudo apt install php8.2-fpm php8.2-cli php8.2-mysql php8.2-curl \
    php8.2-mbstring php8.2-xml php8.2-zip php8.2-gd

Checking the Running PHP-FPM Services

systemctl status php7.4-fpm php8.1-fpm php8.2-fpm

# List of sockets
ls /run/php/
# php7.4-fpm.sock  php8.1-fpm.sock  php8.2-fpm.sock

Nginx Setup: Different Versions for Different Sites

# Site 1 — PHP 8.2
server {
    server_name newsite.com;
    root /var/www/newsite;
    location ~ \.php$ {
        fastcgi_pass unix:/run/php/php8.2-fpm.sock;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    }
}

# Site 2 — PHP 7.4 (legacy site)
server {
    server_name oldsite.com;
    root /var/www/oldsite;
    location ~ \.php$ {
        fastcgi_pass unix:/run/php/php7.4-fpm.sock;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    }
}

Choosing the Default PHP CLI Version

# Show the current version
php --version

# List the available versions
update-alternatives --list php

# Change the default version
sudo update-alternatives --set php /usr/bin/php8.2

# Interactive selection
sudo update-alternatives --config php

Switching the PHP Version in cPanel (via .htaccess)

# In the site's .htaccess
AddHandler application/x-httpd-php82 .php
# or via the PHP-FPM pool — set in the hosting control panel

Each PHP-FPM version runs independently with its own pools and configuration. Changing php.ini for PHP 8.2 does not affect PHP 7.4.

Real Errors From Mixing Versions

Socket not found

connect() to unix:/run/php/php7.4-fpm.sock failed (2: No such file or directory), client: 1.2.3.4, server: oldsite.com, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.4-fpm.sock:"

This is an Nginx error, not a PHP one: the socket named in the vhost does not exist. Either the php7.4-fpm service is not running, or its own pool config names the socket differently — check both places, not just Nginx.

Apache with mod_proxy_fcgi produces the same underlying error with different wording — AH01079: failed to make connection to backend. The cause is the same: the web server is reaching for the wrong socket, or the service is down.

Syntax not supported by the old version

PHP Parse error:  syntax error, unexpected identifier "string", expecting "{" in /var/www/html/app/Status.php on line 5

A typical cause is code using an enum (introduced in PHP 8.1) while the site is mistakenly served by a PHP 7.4 pool. A syntax error with no hint about the version is confusing: the code looks perfectly valid when opened on a developer's machine running PHP 8.2.

When Several Versions Become a Source of Problems

An extension is installed for one version but not another: php8.2-imagick is present while php7.4-imagick is not, and the legacy site fails with "Class Imagick not found" for no obvious reason at first glance.
Each PHP-FPM version keeps its own worker pool and its own OPcache — with limited RAM, three PHP versions consume memory like three independent servers.
Cron jobs launched through the plain php command use the default version from update-alternatives, not the one configured in the site's FPM pool — it is easy to end up with the web and cron on different versions.

Verifying the Result

# Are all the daemons actually running?
systemctl status php7.4-fpm php8.1-fpm php8.2-fpm --no-pager

# Which version is serving a specific site
curl -sI https://newsite.com | grep -i x-powered-by

# Nginx configuration syntax is valid
nginx -t

If systemctl status shows active (running) but the site still returns a 502, check the owner and permissions on the socket file itself: PHP-FPM and Nginx must have access to the same path with matching group permissions.

Another way to confirm which version is actually serving a request is to temporarily add echo phpversion(); to the site's code and compare it with the browser output. It is faster than a full phpinfo(), but good enough for a spot check.

Remove that debug line right after checking — a forgotten echo phpversion(); on a live page leaks the PHP version to anyone, including bots scanning for known vulnerabilities tied to specific releases.